The conventional biological filtration system widely used in aquatic animal breeding environments is based on the nitrogen cycle to transform two toxic nitrogen compounds (ammonium and nitrite) produced by natural biological processes into nitrate, which is a relatively less toxic nitrogen compound. It consists of media (sponge, ceramic, sand) for fixation of nitrifying bacteria and a circulation system that forces the passage of water through these media. The nitrifying bacteria develop spontaneously in the media and transform the ammonia produced by natural decomposition of the organic material present in the environment water and through the secretions of its inhabitants into nitrite and the latter into nitrate, which accumulates dissolved in water. The nitrate is relatively less harmful to the environment and its inhabitants than ammonia and nitrite, but it must be removed before its concentration becomes critical. The amount of nitrate produced in this process is proportional to the quantities of inhabitants and food provided daily, and their removal is performed through periodic partial water changes. The maintenance of the nitrate concentration at acceptable levels depends on the correct balance between these factors. The cultivation of aquatic plants also aids in the removal of nitrate because these consume it to obtain the nitrogen necessary for their development.
The balance between the quantity of inhabitants, quantity of food and the frequency of partial water changes is commonly neglected, resulting in high concentration of nitrate and causing stress on the inhabitants of the environment and death in case of prolonged exposure. The necessary frequency of the partial water changes for nitrate removal, generally weekly or bimonthly, hinders the environmental maintenance. The appearance of opportunist algae that develop due to excess nitrate compromises the esthetics of the environment as well as hinders its maintenance. The cultivation of aquatic plants as an aid in nitrate control has inherent difficulties that do not justify its use only for this purpose.